1. Field of the Invention
The invention relates to reinforced pistons for internal combustion engines.
In general, pistons for internal combustion engines are provided with a ring band which extends around a crown of the piston and in which are formed two or more piston ring grooves. A piston ring is provided in each groove and engages an associated cylinder or liner during operation of the engine. Since the piston rings are commonly made of a ferrous material and the piston is commonly made of a light metal or metal alloy such as aluminium or an aluminium alloy, it is customary to form at least one of the piston ring grooves (usually the uppermost) in a ferrous insert which is cast into, and surrounded by, the light alloy, in order to reduce piston ring groove wear.
There is, extending from above the uppermost piston ring to the level of the crown, a dead volume which is in communication with the combustion chamber formed above the piston. It has been found that, when this dead volume is reduced, there is a corresponding reduction in the exhaust emission and an improvement in the fuel consumption of the associated engine. One previous proposal for reducing this volume has been by a reduction in the clearance between the piston and the associated cylinder or liner. However, in highly rated engines, this can lead to a build-up of carbon in this volume, which causes bore polishing and can lead to high oil consumption.
2. Review of the Prior Art
In view of this, it has recently been proposed to reduce the dead volume by locating the uppermost piston ring as close as possible to the level of the crown. This involves moving the associated piston ring groove insert as high as possible up the piston, even to the extent of having the top of the piston ring groove insert flush with the top of the piston crown.
Although such a design reduces the dead volume to a minimum, it has a potentially very serious failure condition if the insert separates from the remainder of the piston. For this reason, such pistons require very careful handling and inspection to ensure that the connection between the insert and the remainder of the piston is maintained fully effective until the engine is operated. Although this reduces the possibility to a minimum, the potential dangers of separation of the piston and the insert are so great as to prevent widespread acceptance of this proposal, despite the attractions of a reduced dead volume. Even when the insert is overlaid by a thin layer of the light alloy, this potential failure condition still exists, because the strength of the light alloy may not be sufficient to maintain the insert in position.